1. Field of the Invention
This invention relates generally to systems for coating objects by sputtering deposition, and more particularly to an improved sputtering cathode assembly having features which greatly improve the duration of useful or working time afforded before the target material must be replaced.
2. Description of Related Art
The sputtering deposition of materials onto objects to be coated with the sputtered material is well known and includes the process of material removal from a target material by its bombardment with highly energized ions formed after high energy electrons are emitted from the target material by placing a high RF or d.c. voltage between the target and the objects to be coated. These emitted electrons ionize processed gas such as argon placed within a vacuum chamber after it has been substantially evacuated of air up to 100 m Torr vacuum pressure.
The processed gas ions then form a plasma, an electrically neutral association of electrons and positive ions. The plasma is caused by the emitting of electrons from the target material. The plasma ions accelerate and strike the target causing atoms to be ejected from the target material which is then deposited onto the objects having previously been placed within the vacuum chamber.
Ideally, the duty cycle of each fresh target of each cathode assembly should last until a significant portion, i.e. 40% to 45% of the target has been deposited onto objects within the vacuum chamber. However, the duty cycle of the target attached to the cathode assembly is so severe that physical deformities in the target and the supporting structure associated with the cathode assembly rapidly deteriorate, requiring premature replacement of the target material. The expense of reconditioning each cathode assembly, including the target material, is substantial both with respect to maintenance, replacement components and system downtime.
Commercially available sputtering cathode assemblies have a threaded hole or use a HELICOIL E-Z LOK as a threaded insert to attach target clamps against the edge of the target positioned directly atop a copper body or top plate of the cathode assembly. Typically within only hours of operation, the target bolts begin to loosen, the HELICOILS become damaged or the bolt is simply sheered off. When the target must be changed, most of the HELICOILS require replacement or the threaded inserts seize and require replacement, usually causing damage to the copper top plate itself. Special tools are required to disassemble and remove damaged HELICOILS and, if damage to the top plate is sufficiently severe, it must be totally replaced.
Sputtering cathode assemblies require that the target and the cathode body be cooled by water to prevent meltdown or damage to the sputtering magnets contained within the magnet module positioned adjacent the top copper plate. To seal off the water cavity, commercially available cathode assemblies must have the target clamped tightly to the cathode body, leaving little or no room for the target to expand. At low duty cycles of low power density, this issue is not sufficiently severe to represent a reduction in duty cycle. However, at higher power densities, the duty cycle is substantially reduced.
There are two ways that a target may be cooled: (a) by direct cooling wherein the back of the target is used to seal the water jacket or (b) indirect cooling utilizing a thin copper top plate or backing plate against which the target is directly clamped. Even with the indirect technique or method, the target is still required to be tightly clamped to the thin backing plate to prevent leakage of water from the water passageway. Thicker backing plates which are sufficiently strong to withstand warpage during high power input levels interfere with heat transfer and reduces target thickness through the thicker top plate causing excess thermal expansion of the target due to overheating. At low power densities of less than 200 watts/in2 are viable under these cooling techniques. However, in applications that require cathodes to work under a duty cycle of densities of 300 watts/in2 or higher, thermal expansion of the target during the duty cycle causes the target clamps to loosen, leading to water leakage. The tightening of target clamps after this occurs is usually not sufficiently remedial to stop water leakage as the target itself is typically warped as well from the thermal duty cycle.
Under these adverse conditions, commercially available targets will only last about 24 hours at higher power levels whereas, the target contains sufficient material for vacuum deposit which could last at least ten working days otherwise.
Commercially available cathode assemblies use either a silver braised water tube or a female pipe tube threaded into the back or base plate of the cathode body. The silver braised water tube over time and misuse in handling can break and require a costly replacement. The threaded female pipe thread over time will also weaken, causing permanent damage and requiring costly replacement of the entire cathode body base plate.
The present invention addresses and substantially improves upon all of the above-described shortcomings currently being experienced by the operator of sputtering deposition systems to greatly increase the longevity of the duty cycle afforded by each fresh sputtering cathode assembly containing a fresh target. Not only is the operative duty cycle extended, but the reconditioning or refurbishing of each cathode assembly when replacing the mostly expended target is greatly facilitated and requiring substantially less time and replacement costs associated therewith.